High specific strength of near-beta titanium alloys is very advantageous for their application in airframe structures. The major obstacle in building competitive passenger aircrafts is fabrication of structures and selection of materials with good balance of performance and weight. The need for these alloys has been determined by the current trends to increase the size and the weight of commercial aircrafts, which in its turn resulted in the increased section of high-loaded components, such as landing gear and airframe components, with the required uniform level of mechanical properties. In addition to that material requirements have become considerably stricter, i.e. a good combination of high strength and high fracture toughness has become a requirement. Such structures are made either of high-alloyed steels or titanium alloys. Substitution of titanium alloys for alloyed steels is potentially very advantageous, since it facilitates at least 1.5 times weight reduction, increase of corrosion resistance and reduced servicing. These titanium alloys give solution to this problem and can be used in production of a wide range of critical items, including large die forgings and forgings with section sizes over 150 to 200 mm and also semi-finished products having small sections, such as bar, plate with thickness up to 75 mm, which are widely used for fabrication of different aircraft components, including fasteners. Despite advantageous strength behavior of such titanium alloys as compared with steel, their application is limited by processing capability, i.e. by relatively high strain during hot working as a result of lower temperatures of hot working as compared with high-alloyed steels, low thermal conductivity and also difficulty to achieve uniform mechanical properties and structure, especially for heavy-section parts. Therefore, individual methods of processing are required to achieve the prescribed metal quality.
Near-beta titanium alloys Ti-5Al-5Mo-5V-3Cr—Zr are characterized by certain advantages when compared with other titanium alloys, e.g. with Ti-10V-2Fe-3Al. They are less susceptible to segregation, show strength behavior up to 10% higher than that of Ti-10V-2Fe-3Al alloy, have improved hardenability, which enables production of forgings with section sizes exceeding 200 mm (almost twice as high) with the uniform structure and properties, they are also characterized by improved processability. Moreover, alloys of this class demonstrate fracture toughness comparable to that of Ti-6Al-4V alloy with the strength over 1100 MPa, at that strength is 150-200 MPa higher than that of Ti-6Al-4V alloy. These alloys meet the requirements placed to the state-of-the-art aircrafts. For example, one of the advanced aircrafts uses forgings made of the alloy of this class, which weight varies between 23 kg (50 pounds) and 2600 kg (5700 pounds), and length—between 400 mm (16 inches) and 5700 mm (225 inches). A key factor governing the quality of these items is their thermomechanical treatment. The known methods are not capable of yielding the required stable mechanical properties.
There is a known method for processing of titanium alloy billets comprising ingot hot working via its upsetting and drawing at beta phase field temperatures with the strain of 50-60%, billet forging at α+β phase field temperatures with the strain of 50-60% and billet final hot working at β phase field temperatures with the strain of 50-60% with subsequent annealing of a forging at a temperature that is 20 to 60° C. above beta transus temperature (hereinafter BTT) and soaking for 20-40 minutes (USSR Inventor's Certificate No. 1487274, IPC B2IJ5/00, published 10.06.1999).
The known method is characterized by high possibility of underfilling of high and thin ribs of complex-shaped die forgings and high localization of deformation during single hot working of billet at β phase field temperatures with the strain of 50-60%. In addition to that when final hot working of billet is done in β phase field via several heating operations, this inevitably results in considerable growth of grain due to secondary recrystallization, which leads to deterioration of mechanical behavior.
There is a known method of manufacture of bars of near-beta titanium alloys for fastener application, which includes billet heating to the temperature above beta transus in β phase field, rolling at this temperature, cooling down to the ambient temperature, heating of rolled stock to a temperature that is 20-50° C. below beta transus temperature in α+β phase field and final rolling at this temperature (RF Patent No. 2178014, IPC C22F1/18, B21B3/00, published 10.02.2002)—prototype.
A drawback of the known method is its application for rolling of relatively small sections, for which final hot working at (BTT-20) to (BTT-50)° C. is sufficient to achieve the required level of microstructure, and, therefore, the required level of mechanical properties. However, speaking of complex-shaped items with large section sizes (thickness over 101 mm) and large overall dimensions, final hot working with the specified strain in α+β phase field is not enough to obtain homogeneous microstructure and uniform mechanical properties. Moreover, the specified parameters of thermomechanical treatment are not optimized for the manufacture of large die forgings.